Estimations of life history parameters and stock assessment for silver croaker (Pennahia argentata) in offshore waters of southern Zhejiang

While silver croaker (Pennahia argentata) is an economically and ecologically important species in the East China Sea, researches on its life history traits, stock assessment and fishery management are limited in recent years. Given the increasing fishing intensity and great changes of ecosystem, limited studies of P. argentata would lead to misunderstanding of its stock status and difficulties in its conservation and management. Based on samples from the scientific survey in the offshore waters of southern Zhejiang in 2016, this study intends to understand the life history of P. argentata (growth, maturity, and mortality), to evaluate the stock status and to provide management advice. This study analyzed the life history traits based on the widely used and empirical methods, i.e. von Bertalanffy growth function for growth parameters, logistic maturity curve, and Pauly empirical formula for mortality coefficients. The yield per recruitment (YPR) and spawning stock biomass per recruitment (SSBR) models were constructed, considering the influence of natural mortality and selectivity. The asymptotic length of P. argentata was estimated to be 25.36 cm, and the exponential rate was 0.32 in the growth function. The current length of 50% selectivity (13.52 cm) was much smaller than the length of first sexual maturity (17.79 cm). Based on the length-converted catch curve, the total mortality of P. argentata was estimated to be 2.62. The Pauly empirical formula estimated the natural mortality M to be 0.74, while the fishing mortality estimate was 1.88. Results of YPR and SSBR models revealed that the biological reference points F_max and F_0.1 were 3.43 and 0.78, while F_20% and F_40% were estimated to be 0.66 and 0.33, respectively. The current fishing intensity was much higher than F_20%, indicating recruitment overfishing for this stock. Scenarios considering different natural mortality (M=0.54, 0.64, 0.74, 0.84 and 0.94) and different selectivity (age of 50% selectivity: t₅₀=1.47,1.77, 2.07, 2.37 and 2.67), were evaluated in the sensitivity analysis for YPR and SSBR models. When M increased from 0.54 to 0.94, the estimates for the current YPR would decrease from 17.69 to 10.48, and the current SSBR would slightly decrease from 0.82 to 0.57. When the t₅₀ increased, both YPR and SSBR declined in different degrees. Both YPR and SSBR, including the biological reference points, were sensitive to M and selectivity. The stock of P. argentata. in offshore waters of southern Zhejiang has been recruitment overfished in 2016, and both natural mortality and selectivity have substantial influence on the stock assessment and management advices for this fishery. Therefore, it is recommended to reduce the fishing intensity and increase the first capture length in order to improve the conservation and management of P. argentata.